Highly pure aromatic acids, i.e., benzoic acid, isophthalic acid, phthalic acid, terephthalic acid trimellitic acid, trimesic acid, etc., are of great commercial importance and are widely used for production of various polymers and other materials, including plasticizers and surface coatings. Polyesters are typically prepared from terephthalic acid by direct condensation polymerization with a polyalcohol. High-performance amideimide polymers are prepared by reacting trimellitic acid with an aromatic diamine. Reaction of trimellitic acid with alcohols such as 2-ethylhexanol results in plasticizers such as trioctyltrimellitate.
Preparation of aromatic acids, i.e., benzoic acid, isophthalic acid, terephthalic acid, trimellitic acid, trimesic acid, etc., by oxidation of alkyl aromatic hydrocarbons in two or more stages in the presence of a catalyst composed of cobalt, manganese and bromine is known in the art. For example, trimellitic acid from pseudocumene in a 2-stage liquid phase oxidation is reported in Japanese No. 56,002,932; G.B. No. 1406,693; U.S. Pat. No. 4,398,040; U.S. Pat. No. 4,284,523; Japanese No. 56,128,730; Japanese No. 57,167,942; Japanese No. 57,046,976; Belgium No. 902,545; U.S. Pat. No. 4,587,350; Japanese No. 63,066,149; U.S. Pat. No. 4,755,622; U.S. Pat. No. 4,764,639; and U.S. Pat. No. 4,816,601. Terephthalic acid from a mixture of p-xylene and methyl p-toluate in a multi-stage liquid phase oxidation reaction is reported in U.S. Pat. No. 4,269,805.
Despite the use of multi-stage oxidation processes in the oxidation of alkyl aromatic hydrocarbons as reported in the above prior art, prior investigators failed to recognize that increased oxygen concentration to increase selectivity, coupled with reduced reaction temperatures and increased total pressure would increase product yield and improve product purity. Particularly, prior investigators failed to recognize that increased oxygen concentration and increased total pressure, with lowered solvent partial pressure in the early stages of oxidation of alkyl aromatic hydrocarbons, significantly reduced the production of product impurities, particularly high-boiling impurities which are difficult to remove in downstream purification steps.
Aromatic polycarboxylic acids are conventionally producted by liquid phase catalytic oxidation of feedstocks containing a polyalkyl substituted aromatic hydrocarbon, such as xylene. Such liquid phase reaction systems are shown in U.S. Pat. Nos. 3,170,768 and 3,092,658, both to Baldwin. Because the chemical conversion of the polyalkyl substituted aromatic reactant to the aromatic polycarboxylic acid product is exothermic, reaction solvents are typically employed to dissipate the resultant heat of reaction in a reflux loop. The current practice is to produce the aromatic polycarboxylic acid product in a continuous process or system that includes an oxidation reactor equipped with a reflux system. The reactor contents include water, dior trimethyl substituted hydrocarbon reactants, reaction solvent, and a suitable oxidation catalyst for effecting conversion of the reactants to the desired polycarboxylic acid product. The oxidation reactor is also equipped with means of agitating the reactor contents.
In a conventional continuous oxidation process for producing aromatic carboxylic acids from an alkyl aromatic wherein the catalyst is a composition of cobalt, manganese and bromine, and the solvent is a mixture of acetic acid and water, excess heat from the exothermic reaction is removed by utilizing the vaporization of the reaction solvent. Under typical conditions of the reaction, despite a total reaction pressure of several hundred pounds per square inch, oxygen partial pressure can be at a relatively low level because the bulk of the system vapor pressure is caused by the solvent vapor pressure, i.e., of the water and acetic acid. The solvent vaporization typically takes place in the oxidation reactor, and condensation of the vapors emanating from the reaction mixture typically takes place in a series of heat exchangers. The heat exchangers typically physically located above the oxidation reactor allow condensed solvent vapors to be refluxed to the oxidation reactor by gravity.
Removal of the heat of the reaction by solvent vaporization, and subsequent condensation in heat exchangers, to control reaction temperature results in reduction of the partial pressure of oxygen and consequent production of by-product impurities due to a limited supply of oxygen. For example, p-xylene in the presence of a cobalt, manganese and bromine catalyst can be oxidized to compounds such as trimethyl diphenylmethane in the presence of a limited supply of oxygen.
Other methods than using the vaporization of the reaction solvent for removing the excess heat of reaction, and thereby controlling the reaction temperature, have been disclosed in the prior art. For example, Belgian Pat. No. 741,534 teaches use of a heat exchanger wherein the chemical compound to be oxidized is first passed into a reaction zone, then withdrawn into a heat exchanger wherein air is injected. The reaction mixture is then returned from the heat exchanger to the reaction zone after removal of excess heat from the reaction mixture. Efficiency of the heat exchanger is maintained by the injection of air into the heat exchanger to reduce fouling of the reactor and heat exchanger.
U.S. Pat. No. 4,269,805 teaches a multi-stage reactor for oxidation of alkyl aromatics, e.g., a mixture of p-xylene and methyl toluate in a liquid phase reaction mixture with oxygen-containing gases, e.g., air, under pressure and temperature in the presence of oxidation catalyst. An internally disposed cooling conduit system containing a coolant for removing the heat of reaction is provided and includes a group of cooling conduits for each of the reaction chambers. Temperature of the first stage is maintained generally at about 150.degree.-155.degree. C. (302-311.degree. F.) and the temperature in successive stages is increased in increments of about 5.degree.-10.degree. C. Pressures of 3 to 10 bar (45 to 150 psi) are taught for oxidation of p-xylene. The reactor is an elongated horizontal closed tank with multiplicity of neighboring reaction chambers arranged successively from one end to the other end of the tank for containing the liquid reaction mixture at predetermined levels in each chamber. The several reaction chambers in the reactor are under the same pressure which is relatively low.
Many prior art oxidation reactors were originally designed to operate at a predetermined temperature range. For a variety of reasons, including product quality, it is desirable to reduce the reaction temperature to below the temperature ranges previously utilized for the oxidation reaction.
Reduced reaction temperatures tend to reduce undesirable burning losses of the polyalkyl aromatic reactant as well as the solvent. Reduced reaction temperatures have been observed to result in a reduction of undesirable oxidation reaction by-products. Thus, it is desirable to reduce the process temperature range so as to improve product yields and quality while reducing process operating costs.
In the conventional polyalkyl aromatic oxidation process, lower reaction temperatures require a simultaneous reduction in the reactor operating pressure. However, as the reactor pressure is reduced, vapor velocities in the reactor increase with attendant reduction in reactor liquid phase residence times. Pressure drops in overhead piping and heat exchangers increase as well. Consequently, as the reactor temperatures are lower in a conventional polyalkyl aromatic oxidation process, equipment limitations are encountered which require either a reduction in unit throughput or significant capital expenditures for equipment alterations needed to maintain capacity.
As the system total pressure is reduced in a conventional process to achieve the desired lower temperature, the oxygen partial pressure at a given dry basis vent oxygen content is also reduced, which decreases selectivity. Accordingly, it is desirable to provide an improved alkyl aromatic oxidation process that can be operated at relatively low oxidation temperatures and at a relatively high pressure wherein oxygen partial pressure is sufficient to increase selectivity. It is therefore an object of this invention to provide a process for the preparation of aromatic mono- or polycarboxylic acids by oxidation of alkyl aromatic hydrocarbons in the presence of a cobalt-manganese-bromine catalyst in an acetic acid-water solvent wherein formation of by-products and impurities is suppressed by use of high reactor pressures and relatively low process temperatures, and wherein oxygen starvation in the reactor is minimized with consequent improved selectivity and improved product yield.
It is further an object of this invention to provide a batch, a semi-continuous or a continuous process for the preparation of aromatic mono- or polycarboxylic acids by oxidation of alkyl-aromatic hydrocarbons in an acetic acid-water solvent solution in the presence of a cobalt-manganese-bromine catalyst wherein formation of by-products and impurities is suppressed in at least a two-stage oxidation reaction wherein selectivity to mono-acids and mono-aldehydes in the first stage is increased by maintenance of oxygen partial pressure. In the second stage and succeeding stages, the oxidation reaction is continued to complete oxidation. The instant invented process is particularly directed to preparation of aromatic polycarboxylic acids such as terephthalic acid, trimellitic acid, and pyromellitic acid from paraxylene, pseudocumene and durene, respectively.